In the 1930's the U.S. and Canadian governments began public work programs to develop a transportation infrastructure comprising roadways and bridges. The increased weight and numbers of today's trucks compared with design loads that were used for the roads and bridges at the time of construction, combined with aging, environmental conditions and the use of corrosive salts has resulted in deterioration and increasing structural deficiencies. Currently, the U.S. has 542,000 bridges that consume billions of dollars each year in construction, rehabilitation and maintenance. In Canada, there are an estimated 10,000 railroad bridges and 30,000 automobile bridges with 40% of these bridges requiring repair or replacement. A similar situation is said to exist in Europe and Asia. It can be appreciated that other structures, such as, for example, but not limited to, aircrafts, dams and buildings can also suffer from similar structural degradation.
In light of these problems, significant research has been directed over the last few years towards the field of structural health monitoring in order to mitigate potential hazards to the general public. The research has been directed towards improved methodologies in detecting and monitoring structural degradation with an eye towards improving service life and minimizing down time for maintenance. Ongoing monitoring may be used on these structures to control and predict maintenance and replacement costs and also to increase the lifetime and reliability of these structures. For example, structural information gathered on bridges is important in determining whether or not load ratings should be changed, to catch faults early enough so that repairs may be done, or to find structural problems that require the bridge to be replaced.
The current movement towards structural monitoring involves a detection suite of distributed smart sensors which can detect potential construction flaws or structural fatigue to expose a potential hazard to the public. Structures having these sensors are referred to as smart structures. Embedded smart structure technology (actuators and sensors) offers the unique ability to assess structures on demand to determine the current condition of the structure. These sensors may also be designed to monitor specific conditions. For example, these devices can provide event-based information such as the condition of structural integrity after a sudden impact from an earthquake, or continuous measurement of data for a range of strain and damage conditions.
Two main groups of prior art sensors have been developed for use in smart structures. The first group of prior art sensors comprise sensors that require hardwiring and include traditional strain gauges and fiber-optic strain gauges. The traditional strain gauges are made of metal foil and are bonded to the structure. The strain is determined by measuring the resistance of the metal foil or by determining the mechanical resonant frequency of the metal foil. The foil gauges require a physical connection to transmit the information regarding the structural strain as well as a DC signal for providing power for the strain gauge. Fiber-optic strain gauges were developed to address some of the problems associated with traditional strain gauges. Fiber optic strain gauges are embedded into the structure but require a fiber-optic connection to make a measurement. One technique for measuring structural strain uses the center reflectivity wavelength of the optical fiber Bragg gratings. Systems based on both traditional strain gauges and fiber-optic strain gauges result in a series of connected sensors throughout the structure.
Both traditional strain gauges and fiber-optic strain gauges require a link to the outside world. Accordingly, when these sensors are installed in a structure, provisions for this link, such as wires, must be provided. Over time these wires can corrode and compromise the integrity of the monitoring system. In addition, fiber-optic units can be difficult to install and can be subject to temperature drift. Furthermore, when the connections linking these sensor systems break then the monitoring system will not function.
The second group of prior art sensors comprise passive sensors that do not require a physical connection. Passive sensors include acoustic sensors and sensors that employ passive circuits for detecting strain.